1. Technical Field
The present invention relates to methods for testing lenses, and particularly, to a method for testing an optical resolution of a lens and positioning the lens relative to an image sensor.
2. Description of Related Art
The ongoing development in microcircuitry and multimedia technology has made still and digital camera modules popular and widely used. Nowadays, various electronic devices such as cell phones and personal digital assistants are equipped with camera modules. Continuous improvements in the quality and resolution of the camera modules are demanded by consumers.
Lenses and image sensors are critical components in camera modules. The position of a lens relative to an image sensor is very important, particularly for a camera module without an auto-focus function (e.g., the lens and the image sensor are both fixed). A distance between a lens and an object is called an object distance. When a lens is at a position where it optimally projects light to an image sensor (i.e., the lens is at a focus state relative to the image sensor), a distance between the lens and the image sensor is called an image distance. When the quality of a lens is tested, the lens typically goes through at least two optical resolution measurements, each of which is scaled to a modulation transfer function (MTF) value. For example, it may be required that a lens satisfy both a first and a secondary required MTF values for a first and a secondary object distances. That is, it is necessary that each MTF value of the lens for an object at different object distances is not less than the respective required MTF value.
Typical methods for testing the optical resolution involve two imaging objects at two different object distances, or a motor for moving an imaging object to two different object distances. The process is complicated because additional equipment or mechanisms are required to perform the test.
Therefore, a new method for testing a lens is desired to overcome the above-described problems.